Why is that when I open up my file in blender, the reference picture is gone
By default, Blender does not save images as part of the .blend file (this applies to background images or textures used as part of the scene).
To make those images part of the file they have to be packed.
If the images are not packed, blender has to re-load them form the hard drive every time you open the file. If unpacked image files get relocated on a different folder or drive, you'll be presented with nothing (for background pictures) or a pink color instead of textures and images used for compositing.
So you have two options:
1. Pack the images to make a self contained .blend file:


After packing you have to re-save your file.
Keep in mind that the file size will increase when saving images.
NOTE: You can't pack image sequences or video files in a blend file.
2. If you don't pack the images,
then they have to be stored on every computer you use to work on the file, using the correct path so that blender can find it.